A priority system in WoW is a list of abilities, each of which is less important than the last. So for example, if your class has a priority system like this:
Mangle > Rip > Rake > Savage Roar > Shred for combo points > Ferocious Bite
Then mangle is the most important thing on the list and ferocious bite is the least important thing on the list. If you cannot use mangle you move on to the next ability on the list, in this case Rip. You move down the list using the abilities you can. But as soon as a more important ability becomes available, you start from that one again.
Different classes have different priority systems. In each case they have different caveats to the priority system and when abilities are more important than others. This is just a basic overview of what a priority system is. An easy way to remember it is:
a) A priority system is different to a set casting rotation;
b) Each ability on the priority system is less important to use than the last.Read more →